1. Field of the Invention
This invention is directed to a method of producing acetic acid by a reactive distillation process from carbonylation of methanol (MeOH) and/or dimethylether (DME).
The invention concerns in particular improved production of acetic acid from methanol, DME or a combination of these components over a homogeneous catalyst system contained in a distillation column.
The catalyst may be any homogeneous carbonylation catalyst which is soluble in the reaction medium.
2. Description of the Related Art
Conventional acetic acid synthesis is performed in a homogeneous process, where methanol is carbonylated in a liquid, catalytic medium contained in a stirred reactor. Methanol derivatives such as methyl acetate and dimethyl ether may be applied instead of or in combination with methanol. Carbon monoxide reactant is typically introduced at bottom of the reactor and distributed in the liquid. The catalyst system comprises one or more Group VIII metal compounds, preferably rhodium or iridium and a halide promoter, e.g. methyl iodide (MeI).
Beside primary reaction (1) other reactions are taking place in the reaction medium. The most predominant are: EQU MeOH+CO W HOAc (1) EQU H.sub.2 O+CO W H.sub.2 +CO.sub.2 (2) EQU (CH.sub.3)O+H.sub.2 O W 2 CH.sub.3 OH (3) EQU CH.sub.3 OH+HI W H.sub.2 O+CH.sub.3 I (4) EQU CH.sub.3 OH+CH.sub.3 COOH W CH.sub.3 COOCH.sub.3 +H.sub.2 O (5)
Also small amounts of higher acids, primarily propanoic acid, is synthesized in the process.
The presence of water is essential to stabilize the catalyst system. So-called stabilizers can be added to the reaction medium in order to reduce the water concentration. A surplus of CO is required to keep the catalyst system activated and unreacted CO gas is purged from the liquid reaction medium at top of the reactor. CO gas (+inerts and hydrogen synthesized by reaction (2)) stream drives off a fraction of volatile components from the liquid, which is recovered and recycled back to the reaction section.
The acetic acid product is recovered in a liquid product stream from the reactor and separated by flash off from the catalyst containing reaction medium in a down-stream flash vessel operating at a pressure lower than the reactor pressure, typically at about 1-2 bar. The liquid from the flash vessel containing the group VIII metal catalyst is recycled to the reactor by means of pumping.
As acetic acid is the least volatile major compound in the flash medium, the recovery of the acetic acid produced unavoidably leads to the undesired flash off of more volatile components also contained in the flash medium such as water, methyl iodide, methyl acetate, hydrogen iodide and unconverted methanol and dimethyl ether.
In order to recover these components from the product down stream the reaction section, they are separated in several distillation columns and absorbers and returned to the reaction section.
The downstream separation process comprises essentially three steps.
1. Primarily methyl iodide and hydrogen iodide are recovered in a light end column and returned to the reactor. PA0 2. Primarily water, methyl acetate, and remaining methyl iodide and hydrogen iodide are recovered in a dehydration column and returned to the reactor. PA0 3. Primarily propanoic acid and a fraction of acetic acid is withdrawn from the bottom of a heavy end column, in which also the product acetic acid is recovered. PA0 (a) carbonylation of methanol, DME or reactive derivatives thereof in a homogenous catalyst containing solution active in the carbonylation; PA0 (b) at the same time collecting the components taking part in present reactions and stripping off mainly unconverted carbon monoxide, hydrogen and inert gases, leaving the remaining components taking part in present reactions; and PA0 (c) at the same time as (b) distilling off the acetic acid product from at least part of the remaining components taking part in present reactions, and resupplying the remaining components taking part in present reactions thus reduced in acetic acid to the carbonylation step.
Various overhead gases are separated from methyl iodide in an absorption system.
In the dehydration column, hydrogen iodide is formed continuously by hydrolysis of methyl iodide (eq. 4) Eventually, this leads to the formation of a hydrogen iodide/water/acetic acid azeotrope. This azeotrope may be dissociated by addition of small amounts of methanol to the dehydration column.
Essentially, the resulting component effluents from the acetic acid synthesis and purification sections are unconverted carbon monoxide (+gases) and the product acetic acid (+byproducts).
The fact that acetic acid is the least volatile major component in the reaction mixture declines the process economy because of energy consumption and investment in the conventional process layout.